feat: 知识库管理界面（入库监控 + 检索台）

桌面端「知识库」模块从占位变为可用：入库(切块/embedding/Milvus 监控) +
检索调试台(向量召回，带分数与来源)。

- mcp-go: 新工具 kb_search(返回结构化 JSON [{text,score}])；rag.Hit 加 json 标签
- gateway: POST /api/v1/kb/search → kb_search（结构化命中给检索台）
- desktop: lib/api ingestKb/searchKb；新 KbView(左 入库+监控日志 / 右 检索台命中列表
  带 Milvus 来源徽标+分数)；App 接 kb 视图；LeftNav 知识库 ready
- 验证: gateway/mcp-go build✓ + e2e PASS + 前端 build✓；真实浏览器——入库3条→监控
  '已入库3块'；语义查询'存储和搜索向量的组件'→Milvus(0.612)>Neo4j>NATS 排序正确,
  全走真实百炼 embedding(控制面下发)+Milvus

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

sundynix-desktop/frontend/src/shell/LeftNav.tsx

@@ -19,7 +19,7 @@ interface Item {
 const ITEMS: Item[] = [
   { key: "home", label: "工作台", icon: "■" },
   { key: "studio", label: "编排", icon: "◆", group: "BUILD", ready: true },
-  { key: "kb", label: "知识库", icon: "▣", group: "BUILD" },
+  { key: "kb", label: "知识库", icon: "▣", group: "BUILD", ready: true },
   { key: "report", label: "报告", icon: "▤", group: "BUILD" },
   { key: "runs", label: "运行", icon: "▸", group: "RUN" },
   { key: "memory", label: "记忆", icon: "◇", group: "MANAGE", ready: true },